The present invention relates to the field of chemical vapor deposition systems, and more particularly, to apparatus and methods for cleaning the residue left by the process gas which has been injected into a plasma chamber of the system.
Chemical vapor deposition (CVD) systems normally employ a chamber in which gaseous chemicals react. From these reactions, a substance is deposited on a wafer surface to form dielectric, conductor, and semiconductor film layers that constitute an integrated circuit, for example. In a chemical vapor deposition system, a process gas is injected into the plasma chamber in which a plasma is formed. Due to the ion bombardment within the plasma of the process gas, (SiH4(silane), for example) silicon will be deposited on a wafer which has been previously placed in the chamber. During this deposition step, the gas injection ports, also known as jet screws, typically clog with silicon-rich oxide residue formed by the combined SiH4 (the process gas) and oxygen radicals flowing to the gas injection port. These oxygen radicals originate from the plasma chamber.
The residue coats the walls of the chamber, and also tends to clog the gas injection ports. The chamber, as well as the gas injection ports, needs to be cleaned periodically. This ensures that each wafer encounters the same environment so that the deposition process is repeatable. Since opening up the chamber (changing out the hardware) for cleaning is very labor intensive and costly, a method for removing the deposition from the chamber walls without opening the chamber itself has been previously developed. This xe2x80x9cinsituxe2x80x9d cleaning has been accomplished in the past using fluorine. The fluorine is injected into the chamber as NF3. Fluorine is known to etch silicon and silicon dioxide at high rates when it is accompanied by ion bombardment. Radio frequency (RF) power provides the energy for ion bombardment, with the NF3 serving as the source of fluorine.
Typically, after a wafer is processed through deposition in the CVD system, the wafer is removed to a load lock. A cover wafer is then transferred to the chamber and placed on the chuck. The cover wafer is a standard silicon wafer that is coated with aluminum. It protects the chuck surface from the plasma cleaning and conditioning steps that follow.
The RF power is applied to the chamber and NF3 is injected into the chamber. The walls will then be cleaned of oxide deposition. However, there may still be a significant amount of fluorine in the chamber and on the walls and free particles. For this reason, a pre-deposition conditioning step is often required. The conditioning step is essentially a deposition that getters the fluorine and tacks down particles onto the chamber walls. When this pre-deposition conditioning step is completed, the cover wafer is transported back to its cassette and the next wafer can then be processed.
In conventional systems for routing the gas to the chamber, injection ports are shared between the deposition process gas (SiH4) and the insitu cleaning gas (NF3). Such an arrangement is shown in prior art FIG. 1 in which a portion of a process chamber is schematically depicted. The plasma chamber 10 injects oxygen at port 12 into the interior 14 of the plasma chamber. The oxygen radicals are formed within the plasma chamber 14. The shared injection ports for the deposition process gas and the insitu clean gas are depicted as reference numeral 16. During the deposition step, the gas injection ports (also known as xe2x80x9cjet screwsxe2x80x9d) clog with silicon-rich oxide residue formed by the combined SiH4 and the incoming oxygen radicals originating from the plasma chamber.
As stated earlier, the insitu cleaning gas is designed to chemically etch the SiO2 (silicon dioxide) residue. However, high pressure caused by supersonic gas flows in front of the jet screws causes regions of scarce fluorine radicals that reduce fluorine induced etching of the SiO2. FIG. 2 a schematic depiction of a detail of a jet screw. NF3 gas is injected into the chamber 14 through the jet screw 16. Within the jet screw, there is SiO2 clogging, schematically depicted at point 18 at the jet screw 16. The high pressure region 20 of scarce fluorine radicals caused by the supersonic gas flows in front of the jet screws 16 reduces the fluorine induced etching of the SiO2 in this area, and in particular, prevents the jet screws 16 from being unclogged of the SiO2 residue. All of the other chamber surfaces are typically cleaned except for the jet screw ports.
Due to the SiO2 clogging of the jet screw ports, the jet screws are normally replaced after approximately 300 wafers have been processed. This process involves shutting down the chamber at high expense and loss of productivity. Another problem of the prior art arrangement is that the SiH4 and NF3 gases, if combined, are highly combustible so that routing the gases through the same injection ports can be relatively dangerous.
There is a need for a gas routing system and method for routing gas in a plasma chamber so as to unclog the jet screws through which deposition gas is injected into the chamber.
These and other needs are met by the present invention which provides an arrangement for insitu cleaning of a chamber in which process gas is injected into the chamber through gas injection ports. The arrangement comprises a chamber in which a process is performed, and at least a first gas injection port in the chamber through which the process gas is injectable into the chamber. At least a second gas injection port is provided in the chamber through which insitu cleaning gas is injectable into the chamber. The cleaning gas injected into the chamber also contacts the first gas injection port to clean the first gas injection port. The first and second gas injection ports are separate ports.
By re-routing of the cleaning gas through a separate, second gas injection port, the pressures within the jet screws are equalized with the pressure of the chamber. This allows higher fluorine dissociation and SiO2 etching.
Another advantage of the present invention is the injection of the SiH4 and NF3 gases through completely separate manifolds, thus providing a clear safety advantage.
A further advantage of the present invention is the reduction in the amount of maintenance required of the chamber. For example, using the gas routing system of the present invention, the chamber does not need to be maintained for approximately 3,000 wafers. This is a decided advantage over the prior art in which the jet screws needed to be replaced after only 300 wafers.
Another advantage of the present invention is that the insitu clean time is decreased due to more efficient cleaning of the jet screws. This provides a throughput advantage of, for example, two wafers per hour. Finally, another advantage is that plasma to surface arcing is completely eliminated in the jet screw area.
Another embodiment of the present invention satisfies the earlier stated needs by providing a method of routing gas to a plasma chamber comprising the steps of: injecting process gas into the plasma chamber through a first gas injection port, and injecting chamber gas into the plasma chamber through a second gas injection port. The second gas injection port is separate from the first gas injection port. The cleaning gas cleans the plasma chamber and the first gas injection port.
Another method of the present invention provides for unclogging jet screw ports in the chamber. The jet screw ports inject process gas into the chamber. This method comprises the steps of terminating injection of the process gas into the chamber and injecting cleaning gas into the chamber through openings separate from the jet screw ports to equalize pressure of the cleaning gas within the jet screw ports with pressure of the cleaning gas within the chamber.
In another embodiment of the present invention, an electron cyclotron resonance chemical vapor deposition system is provided. This system includes an electron cyclotron resonance plasma chamber, and a gas supply that supplies plasma forming gas, process gas, and cleaning gas to the plasma chamber. The plasma chamber has a first port through which the process gas is supplied, and a second port, separate from the first port, through which the cleaning gas is supplied.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.